Intracavity laser spectroscope for high sensitivity detection of contaminants

ABSTRACT

Contaminants are detected optically at concentrations below 1 part-per-million (ppm) and extending to a level approaching 1 part-per-trillion (ppt) by using intracavity laser spectroscopy (ILS) techniques. A solid-state laser with an ion-doped crystal medium contained in an optical resonator cavity (the ILS laser) is employed as a detector. A gas sample containing gaseous contaminant species is placed inside the optical resonator cavity and on one side of the ion-doped crystal. The output signal from the ILS laser is detected and analyzed to identify the gaseous species. The concentration of the gaseous species can be determined as well.

TECHNICAL FIELD OF THE INVENTION

This invention relates, generally, to the detection of contaminants ingases, and more particularly, to the high sensitivity detection ofgaseous molecules, atoms, radicals, and/or ions by laser techniquesgenerally termed intracavity laser spectroscopy (ILS).

BACKGROUND OF THE INVENTION AND PRIOR ART

In the preparation of high quality semiconductor material (e.g. siliconfilms) for use in the microelectronics industry it is well known thatcontaminants must be controlled. Failure to control contaminants, as isalso well known and appreciated, can result in the loss of significantresources as the resultant products are typically not useful for theirintended purposes.

Generally, the starting materials in the fabrication of silicon filmsconsist essentially of gases, typically denoted either "bulk" (e.g.nitrogen or argon) or "specialty" (e.g. hydrogen chloride, hydrogenbromide, boron trichloride). The successful operation of a fabricationfacility designed to prepare semiconductor materials depends directly onthe purity of the starting gases, as well as the gas handlingcapabilities available to preserve gas purity during the delivery of thegases to the process chamber and while material processing is takingplace. Suitable control of the purity of such starting gases (i.e.monitoring and inhibiting high levels of contaminants as may becontained in the gases) is essential.

Under many current techniques, such control is achieved after the fact.That is, the silicon films so produced are periodically tested and theproduction line shut down only after such tests reveal the presence ofhigh level contaminants. These processes, as will be appreciated bythose skilled in the art, can lead to the waste of not only startingmaterials but also product which is produced prior to cessation ofproduction. It is therefore desirable to monitor and control thecontaminants as may be contained in such starting gases duringproduction so as unacceptable contaminant levels are observed,production can be immediately, or at least shortly thereafter halted.

Many molecular, atomic, radical, and ionic species are present in thebulk and specialty gases used in the preparation (e.g. chemical vapordeposition or "CVD") and processing (doping and etching) ofsemiconductor materials that can be viewed as "contaminants." Suchcontaminants can degrade either the quality of the fabricatedsemiconductor material or the efficiency with which the semiconductormaterial is prepared. These contaminant species can interfere with thechemical process directly or even cause particles to be formed in thegas delivery lines or process chamber which subsequently deposit on thesurface of the wafer material causing indirect performance defects.

The first step in controlling and/or eliminating these contaminants istheir detection in the bulk and specialty gases used as startingmaterials. While this is generally recognized, heretofore practicedmethods are generally inadequate. This is due, in large part to thesituation created by seemingly ever increasing competitive industrystandards which have developed. Specifically, as the size ofmicroelectronic devices has decreased while performance specificationshave been intensified, the requirements for gas purity (i.e., absence ofmicrocontamination) has increased.

Against this backdrop it will likely be clear that several measurementcriteria are important to detector effectiveness: (1) absolute detectionsensitivity usually stated as parts-per-total number of gas molecules inthe sample (e.g. parts-per-million or number of contaminant moleculesper 10⁺⁶ background molecules); (2) species selectivity or thecapability to measure the concentration of one species in the presenceof other species; (3) rapidity of measurements to obtain a desiredsignal to noise ratio; (4) capability of monitoring contaminants in bothnon-reactive and reactive gases; and (5) linearity and dynamic range ofgas concentrations that can be measured.

The current state-of-the-art devices for contaminant detection (e.g.,water) encompass a variety of measurement techniques. For example,current state-of-the-art devices for water vapor detection utilizeconductivity and electrochemical, direct absorption spectroscopy, andatmospheric pressure ionization mass spectroscopy (APIMS) measurementtechniques. As discussed below, each of these methods falls toadequately address these requirements.

CONDUCTIVITY AND ELECTROCHEMICAL

Conductivity and electrochemical methods by solid-state devices existwhich can detect water vapor at the 1-100 ppm range. Conductivity andelectrochemical methods generally require direct physical contactbetween the sample and the device; thus, detection occurs after watermolecules deposit on the solid-state surface. As a consequence, thesedevices do not perform well, if at all, with utilization of reactive orcorrosive gases. Indeed, even their performance in non-reactive gaseschanges and/or deteriorates after even short exposures to reactive orcorrosive gases. The linearity and dynamic range of response are usuallylimited to about one decade. The detection selectivity of these deviceswith respect to different gaseous species also is generally poor sincethe devices themselves will respond to a wide range of species withoutdiscrimination and selectivity is incorporated into the measurementsonly through whatever chemical selectivity, if any, is embodied in thecoatings used to cover these devices.

DIRECT ABSORPTION

Direct absorption spectroscopy generally relates to the passing of lightthrough the sample from an external source and measuring the reductionin light intensity caused by molecular, atomic, radical, and/or ionicabsorption in the sample. Detection sensitivity depends directly on thesubtraction of two large numbers (light intensity from the externalsource before it passes through the sample and its intensity after itexits the sample). This limits the detection sensitivity to the extentthat direct absorption is generally considered a low sensitivitymethodology.

APIMS

APIMS, initially used in the analysis of impurities in bulk nitrogen andargon and ambient air for air pollution studies, is now currently usedby semiconductor manufacturers to detect trace levels of moisture andoxygen in inert bulk gases. With APIMS, the sampled gas is bombardedwith electrons, or may be flame and photon excited, to produce a varietyof ions that are then detected directly. Particularly, ionization occursat atmospheric pressure in the presence of a reagent gas in theionization source. APIMS typically exhibits detection sensitivities inthe range of about 10 parts per trillion (ppt) in non-reactive gases.APIMS cannot even be used with reactive gas mixtures. Additionaldisadvantages of APIMS include $150,000-250,000 cost, extensive purgingand calibration procedures, and the need for a knowledgeable operator.

In the context of the present invention, laser technology, specificallyintracavity laser spectroscopy (ILS) is disclosed as being used as adetector (sensor) to detect gaseous species (contaminants) at very highsensitivity levels. While the methods and apparatus disclosed herein areparticularly suited for application in fabrication of semiconductorcomponents, it should be appreciated that the present invention in itsbroadest form is not so limited. Nevertheless, for convenience ofreference and description of preferred exemplary embodiments, thisapplication will be used as a benchmark. In connection with thisapplication, laser technology offers distinct advantages to gaseousspecies (contaminant) detection over known methods and, particularly, towater vapor detection.

In conventional applications of lasers to the detection of gaseousspecies (contaminants), laser produced radiation is used to excite thegas sample external to the laser in order to produce a secondary signal(e.g. ionization or fluorescence). Alternatively, the intensity of thelaser after it passes through a gas sample, normalized to its initialintensity, can be measured (i.e., absorption).

Intracavity laser spectroscopy (ILS) combines the advantages ofconventional absorption spectroscopy with the high detection sensitivitynormally associated with other laser techniques such as laser-inducedfluorescence (LIF and multiphoton ionization (MPI) spectroscopy. ILS isbased on the intracavity losses associated with absorption in gaseousspecies (e.g. atoms, molecules, radicals or ions) found within theoptical resonator cavity of a multimode, homogeneously broadened laser.These intracavity absorption losses compete via the normal mode dynamicsof a multimode laser with the gain generated in the laser medium.Traditionally, ILS research has been dominated by the use of dye lasersbecause their multimode properties fulfill the conditions required foreffective mode competition and their wide tunability provides spectralaccess to many different gaseous species. Some ILS experiments have beenperformed with multimode, tunable solid-state laser media such as colorcenters and Ti:Sapphire. D. Gilmore, P. Cvijin, G. Atkinson,"lntracavity Absorption Spectroscopy With a Titanium: Sapphire Laser,"Optics Communications 77 (1990) 385-89.

ILS has also been successfully used to detect both stable and transientspecies under experimental conditions where the need for high detectionsensitivity had previously excluded absorption spectroscopy as a methodof choice. For example, ILS has been utilized to examine gaseous samplesin environments such as cryogenically cooled chambers, plasmadischarges, photolytic and pyrolytic decompositions, and supersonic jetexpansions. ILS has been further used to obtain quantitative absorptioninformation (e.g. line strengths and collisional broadeningcoefficients) through the analysis of absorption lineshapes. Some ofthese are described in G. Atkinson, "lntracavity Laser Spectroscopy,"SPIE Conf., Soc. Opt, Eng. 1637 (1992)

Some twenty years ago, another detection methodology was first exploredin which the laser itself is used as a detector. G. Atkinson, A. Laufer,M. Kurylo, "Detection of Free Radicals by an Intracavity Dye LaserTechnique," 59 Journal of Chemical Physics, Jul. 1, 1973. These methods,while suitable for use in laboratory settings are unacceptable forcommercial settings. The constraints of commercial reality, as brieflynoted above, essentially dictate that such a detector be convenientlysized, relatively inexpensive and reliable. Laboratory models fail tofully meet these requirements.

A laboratory demonstration of the feasibility of using ILS techniquesfor detecting small quantities of water vapor in a nitrogen atmospherewith a Cr⁴⁺ :YAG laser is described in D. Gilmore, P. Cvijin, G.Atkinson, "lntracavity Laser Spectroscopy in the 1.38-1.55 um SpectralRegion Using a Multimode Cr⁴⁺ :YAG Laser," Optics Communications 103(1993) 370-74. The experimental apparatus utilized was satisfactory fordemonstration of operational characteristics, but undesirable forimplementation in a commercial application as contemplated by thepresent invention.

In accordance with various aspects of the present invention, the presentinvention provides a user friendly, i.e. comparatively simple, detectionsystem, having the advantages of direct absorption techniques but withdramatically increased detection sensitivities, capable of detectinggaseous species in reactive and non-reactive samples at a commerciallyviable cost, In this regard, the present invention addresses the longfelt need for a method and apparatus for the high sensitivity detectionof contaminants in reactive and non-reactive gas systems in commercialsettings,

SUMMARY OF THE INVENTION

In accordance with various aspects of the present invention,contaminants are detected optically at concentrations below 1part-per-million (ppm) and extending to level approaching 1part-per-trillion (ppt) by using ILS techniques. A solid-state laserwith an ion-doped crystal medium and operating in the 1300 nm to 1500 nmspectral region preferably serves as the detector. The gas samplecontaining gaseous contaminant species, for example water vapor, isplaced inside the optical resonator cavity of the ion-doped laser(between reflective surfaces or mirrors) and on one side of the activemedium. Laser media having Cr⁴⁺ :YAG and Cr⁴⁺ :LuAG are described here,but other ion-doped crystals having multiple longitudinal and transversecavity modes can be used as well. For example, a Ti:Sapphire laser maybe optically configured to provide ILS detection of oxygen and watervapor.

The ILS water vapor sensor system preferably comprises a pumping laserused to provide the optical excitement required to operate the ILSlaser, a multimode ILS laser operated over the wavelength region inwhich the species of interest absorb, a gas sample cell placed withinthe optical resonator cavity of the ILS laser, a modulating devicedesigned to periodically interrupt the intensity of the pumping laserbeam and the output from the ILS laser, a wavelength dispersivespectrometer capable of spectrally resolving the output of the ILSlaser, a detector capable of measuring the wavelength-resolved intensityof the ILS laser output, and an electronic circuit which can read thesignal from the multichannel detector and convert it into electronicsignal that can be processed by a computer.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred exemplary embodiments of the present invention will behereinafter described in conjunction with the appended drawing figures,wherein like designations denote like elements, and:

FIGS. 1A and 1B are schematic block diagrams of a contaminant detectorsystem in accordance with the present invention, FIG. 1A shows the basicconfiguration, while FIG. 1B shows that configuration as embodied in thepreferred embodiment shown in FIG. 2;

FIG. 2 is a more detailed schematic perspective view of a preferredembodiment of a contaminant detector system in accordance with thepresent invention;

FIG. 3 is a schematic perspective view of an ILS chamber including thechamber components depicted in FIG. 2, some of the components shown inpartially broken away fashion;

FIG. 4 is an enlarged perspective view of a preferred exemplaryembodiment of a beam shaping assembly including a chopper element whichmay be advantageously used in connection with the contaminant detectorsystem shown in FIG. 2;

FIG. 5 is a perspective view of an exemplary ILS laser crystal holderand heat sink useful in connection with the contaminant detector systemshown in FIG. 2;

FIG. 6 is a graph showing an exemplary water absorption spectrum in N₂and HCI gases over the wavelengths of 1433-1440 nm;

FIG. 7 is an ILS water absorption in N₂ and HCI gases over thewavelengths of 1420-1431 nm;

FIG. 8 is a graph showing water absorption intensity versus waterconcentration as determined by permeation tube/volume expansiontechniques;

FIG. 9 is a graphical depiction of water absorption intensity versuswater concentration also in the 1420-1430 nm region as determined by anin-line purifier;

FIG. 10A-10C includes schematic representations of simple laser devices,and

FIGS. 10D-10F are graphs which represent the accompanying graphicalspectral outputs (intensity versus wavelength) obtainable from thedevices depicted in FIGS. 10A-10C, respectively.

DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS OF THE PRESENTINVENTION

As previously briefly noted, the subject matter of the present inventionis particularly well suited for use in connection with the manufactureof semiconductor components, and thus, a preferred exemplary embodimentof the present invention will be described in that context. It should berecognized, however, that such description is not intended as alimitation on the use or applicability of the subject invention, butrather is set forth to merely fully describe a preferred exemplaryembodiment thereof.

In this regard, the present invention is particularly suited fordetection of contaminants. Contaminants as used herein refers tomolecular, atomic, radical and ionic species such as may be present ingaseous materials, such as in the gaseous materials which are used inthe fabrication of silicon films, i.e. inlet lines. Alternatively, theterm contaminant may also refer to the gaseous material itself, such as,for example when the detector of the present invention is used todetermine if a line (e.g. HCI line) has been sufficiently purged of thegaseous material.

In accordance with a preferred embodiment of the present invention andwith momentary reference to FIG. 1A, a gas (contaminant) detector system10 suitably comprises a pumping laser system A, an ILS laser andassociated chamber B, a spectrometer C and a detector with associatedelectronics (e.g. computer, etc.) D. More particularly, and withreference to FIGS. 1B and 2, pumping laser system A suitably comprises apumping laser 100, a beam shaping optics assembly 200 and a beammodulation assembly 300; laser and chamber B suitably comprises achamber assembly 400 and an ILS laser 500; spectrometer C suitablycomprises a spectrometer assembly 600; and, detector D suitablycomprises a detector assembly 700 and a computer system 800. As will bedescribed more fully herein, gas detector system 10 advantageouslydetects gaseous species (contaminants) which are suitably contained in agas sample. In general, pumping laser driver system A suitably pumps ILSlaser B, preferably at or near the threshold level such that a laserbeam passes through the gas sample such that the spectrum of the gassample may be obtained. This spectrum is suitably detected through useof detector/computer system D which, upon manipulation, enables thereliable and accurate determination of the presence and concentration athigh sensitivity levels of gaseous species (contaminants) which may becontained within the gas sample.

With reference to FIG. 10, and in order to more fully explain thescientific principles utilized in accordance with a preferred embodimentof the present invention, the general principles of intracavity laserspectroscopy (ILS) are illustratively shown. As is known, in itssimplest terms, a laser can be described as containing a gain medium, inwhich optical gain is produced, and a resonator, comprised of opticalelements such as mirrors. Optical losses may appear in both the mediumand the optical elements comprising the laser cavity (e.g. theresonator). With particular reference to FIG. 10A, a laser device in itssimplest form can be schematically illustrated as including a gainmedium 1A around which respective mirrors 2A and 3A are placed. Mirrors2A and 3A are typically coated to have high reflectivity surfaces over abroad spectral range. For example, the mirror coating on mirror 2A maybe totally reflective, while the mirror coating on mirror 3A may bepartially reflective thereby permitting some light to escape from thelaser cavity. The spatial region between the reflective surfaces ofmirrors 2A and 3A in which the gain medium is placed defines the laserresonator or cavity, and in the context of the present invention relatesto the so-called "intracavity region."

The intensity (I) of the laser output may be determined both by thewavelength region over which the gain medium operates (λ) and thereflectivity of the resonator elements (e.g. mirrors 2A and 3A).Normally this output is broad and without sharp, distinctive spectralfeatures, as is shown in the plot of I versus wavelength (λ) provided inFIG. 10D of FIG. 10.

By selecting different optical elements to form the laser cavity, thespectral output of the laser can be altered or "tuned." For example, andwith continued reference to FIG. 10 and in particular schematicrepresentation 10B thereof, a tuned resonator cavity may include adiffraction grating 2B which replaces the highly reflective mirror 2Ashown in FIG. 10A. As shown, the laser device therefore includesdiffraction grating 2B, mirror 3B and a medium 1B positionedtherebetween. In general, the result in spectral output from this tunedlaser will be narrowed and appear as wavelengths within the originalspectral output of the laser defined by the gained medium and themirrors (FIG. 10A). For example, a schematic plot of intensity (I)versus wavelength (λ) illustrating a narrowed output is depicted in FIG.10E.

The laser output can also be altered by placing gaseous molecules,atoms, radicals and/or ions in either their ground or excited statesinside the optical resonator (e.g. cavity). With reference to FIG. 10C,a laser so configured may include a highly reflective mirror 2C, apartially reflective mirror 36 with a medium 1C and an intracavityabsorber 4 placed therebetween. In this case, intracavity absorber 4 maycomprise such gaseous species (e.g. the sample containing contaminants).The effect of the intracavity gaseous species on the laser output can beobserved. For example, a plot of I versus λ for such a device is shownin FIG. 10F. FIG. 10F comprises an absorption spectrum of the gaseousspecies contained within intracavity absorber 4. The distinct absorptionfeatures illustrated in FIG. 10F arise from the intracavity specieslosses against which the laser gain must compete. Thus, the absorptionspectrum of the intracavity species may appear in the spectral output ofthe laser. In particular, the laser output intensity (I) at wavelengthswhere the stronger intracavity absorption features compete effectivelyagainst the gain properties of the resonator are more reduced. As aresult, as illustrated, instead of a relatively smooth continuousoutput, such as shown in FIG. 10D, a structured laser output such asshown in FIG. 10F may be observed. The decreases in intensity (I), asshown in FIG. 10F, are due to absorption by the gaseous intracavityspecies, i.e. the more intense the absorption features, the larger thedecrease in the laser output intensity. In accordance with the presentinvention, the absorption spectrum obtained by intracavity lasermeasurements in which an intracavity absorber is employed can beutilized for the high sensitivity detection of such gaseous species. Ithas been found that each gaseous species can be uniquely identified byits respective absorption spectrum (signature) and thus can be used toconfidently identify such gaseous species (contaminant).

The present inventors have found that the appearance of the absorbingspecies (gaseous elements) within the laser resonator before and/orduring the competition between gain and losses which naturally occur asthe laser system approaches threshold give rise to enhanced detectionsensitivity through use of ILS. In view of the fact that the lossesassociated with the intracavity absorber become part of the competitionbetween the gain and losses within the laser, even a small absorbanceassociated either with a weak absorption transition and/or an extremelysmall absorber concentration is amplified dramatically during thegain/loss competition. As a result, such competition clearly appears inthe output of the ILS signal (see FIG. 10F). Thus, using theseprinciples, ILS can be utilized to detect both weak absorption and/orextremely small absorber concentrations.

Against the backdrop of these general principles, in the context of thepresent invention, the present inventors have devised a commerciallyviable contaminant sensor system 10 which provides enhanced detection ofcontaminants in gaseous samples. With reference now to FIGS. 1A and 2,and in accordance with a preferred exemplary embodiment of the presentinvention, a detection system 10 suitably includes laser driver 100, anILS chamber assembly 400 in which ILS laser 500 is contained.Spectrometer 600 and a detector/computer system 700,800 are suitablyoptically connected to the output from laser 500 whereat the absorptionspectrum is suitably manipulated thus enabling the high sensitivitydetection of the presence and/or concentration of gaseous species(contaminants).

In order to drive ILS laser 500, system 10 requires a pumping sourcewhich delivers radiation of sufficient power and within a suitablewavelength region so as to optically excite ILS laser 500 at or slightlyabove its threshold. In this regard, it is important that ILS laser 500operate such that the gain in the laser medium exceeds the overalloptical losses, including those associated with the gain medium,mirrors, and non-mirror intracavity optical elements, as well as theabsorption of any gaseous species within the optical resonator cavity.Moreover, preferably laser 500 operates with multiple longitudinalmodes, i.e. over a broad wavelength region. Typically, a desirablebandwidth over which laser action occurs is between about 2 nm and about15 nm. While ILS laser 500 can also operate with more than onetransverse resonator mode, such is not necessary. Thus, in accordancewith a preferred exemplary embodiment of the present invention, thelaser driver comprises an optical pumping laser 100. Suitably, theoptical parameters (e.g. average power density, peak power density,divergence and beam diameter) of pumping laser 100 advantageously matchthe optical requirements of ILS laser 500. As will be appreciated, to doso it is necessary to determine how many photons can be delivered withina specific volume and at a given distance from the pumping laser over aparticular period of time. In general, in accordance with the presentinvention, such determinations are made in accordance with knowntheoretical and quantitative equations such that the pumping laser issuitably selected to advantageously match the optical characteristics ofILS laser 500.

While other drivers may be utilized in the context of the presentinvention, preferably, a pumping laser is selected on the basis of itsoperational wavelength and on its optical parameters in a manner suchthat it can alone be used to excite ILS laser 500. As will be describedin greater detail hereinbelow, in cases where pumping laser 100 is noteffective alone to drive (e.g. pump) ILS laser 500, beam modificationoptics, such as beam shaping assembly 200, can be utilized. However, inthose cases where the radiation emanating from laser 100 suitablymatches the mode volume of the ILS gain medium contained within ILSlaser 500, such beam modification optics are unnecessary.

In accordance with a particularly preferred aspect of the presentinvention, pumping laser 100 suitably comprises a laser operating atapproximately a 1064 nm wavelength, having an output power greater thanabout 2.8 watts with a TEM₀₀ transverse mode structure. Suitably, a beamE propagating from pumping laser 100 has a linear polarization and isrotatable perpendicular to the plane of propagation. Preferably, thedivergence of the output beam (beam E) from pumping laser 100 is on theorder of less than 0.5 mrad and evidences a beam diameter on the orderof less than 5 mm. A particularly preferred pumping laser is modelT20-1054C from Spectra Physics of Mountain View, Calif. As will beexplained more fully hereinbelow, use of such a pump laser 100 typicallyrequires use of beam shaping optics assembly 200.

It should be appreciated that driver 100 may comprise any suitableoptical pumping source, either coherent or incoherent, continuous orpulsed, that will suitably excite ILS laser 500. As a result, even inaccordance with the previously recited preferred embodiment, pumpingsource 100 operates in a conventional manner and emits radiation over adesired frequency band and having a desired bandwidth.

With continued reference to FIG. 2, ILS laser 500, in the simplest casecomprises an optical resonator cavity defined by the entire optical pathlength between respective mirrors 501,503,505. In those cases wheresystem 10 is used to detect gases (contaminants) within the sample whichdo not chemically react with the components of the laser itself (e.g.gain medium or crystal, mirrors, mechanical mounting and the like), theresonator cavity can be defined by the region between mirrors 501,503,and 505. In such a case, the gas sample region comprises the regionbetween mirrors 501,503,505 (excluding the laser crystal 507).

However, for samples which do chemically react (e.g. a corrosive gas)with one or more of the laser components, it is desirable to separatethe gas sample region from such components. In accordance with apreferred embodiment of the present invention, a separate sample system400A may be advantageously utilized to isolate the sample from the lasercomponents.

With reference to FIGS. 2 and 3, in accordance with this preferredaspect of the present invention, sample system 400A preferably comprisesa gas sample cell body 406 suitably maintained within a gas sample cellholder 407. Respective cell windows 404 and 405 are suitably mounted onthe distal ends of gas sample cell body 406 and provide optical accessto the sample within cell body 406. Windows 404 and 405 also suitablyseal cell body 406. As will be discussed in greater detail below, theregion in which system 400A is suitably placed is astigmaticallycompensated. Given this astigmatic compensation, windows 404 and 405 donot have large effects on the ILS laser beam except with respect totransmission. An inlet conduit 408 and an outlet conduit 409 areoperatively connected to gas cell body 406. With reference to FIG. 3,couplings 408 and 409 are advantageously employed to ensure efficientand effective passage of a gas sample into and out of gas (contaminant)sample cell system 404-409.

Suitably, cell body 406 comprises a stainless steel body havingdimensions suitably on the range of 10 millimeters (mm) to 90 mm. Inaccordance with a particularly preferred aspect of the presentinvention, sample system 400A defines an opening (i.e. the opening inbody 406) having a diameter on the order of about one-eighth of an inch;preferably the opening in body 406 is symmetrically in the center of gassample cell body 406. Preferably, the diameter of the opening in cellbody 406 is suitably selected to be significantly larger than thediameter of the incoming beam such that optical alignment of gas samplesystem 400A may be easily obtained. Windows 404, 405 may be formed anoptically compatible material, such as Infrasil™ available from ResearchElectro Optics of Boulder, Colo.

In accordance with a preferred aspect of the present invention, thethickness of windows 404, 405 is suitably selected to avoidinterferometric effects which may interfere with the quality of the ILSabsorption spectrum obtained through operation of detector 10. Inaccordance with this aspect, the material used in forming windows 404,405 is optimally chosen to minimize absorption losses in the region overwhich ILS laser 500 operates, such as in the range of 1350 nm to 1550nm. Windows 404, 405 are suitably oriented at Brewster's angle so as tofurther minimize reflective losses from the window surfaces.

As so configured, gas sample cell 404-406 suitably permits beam F topass through a gaseous sample to be analyzed. Couplers 408, 409, aresuitably selected to provide easy adjustment such as may be required torealign and/or align windows 404, 405 within ILS laser 500 withoutsignificantly altering the threshold pumping conditions. The resonatorcavity in the case where system 400A is employed is suitably defined bythe physical length between mirrors 501,503,505 (including the lasercrystal 507 and including the region between windows 404,405 as well aswindows 404 and 405 themselves that comprise the sample system 400A).However, in such cases where the sample does not chemically react withthe laser components, the sample cell may nominally be defined by thephysical region between mirrors 501,503,505 (excluding laser crystal507).

In the event that system 400A is present within chamber 400, it isnecessary that any gases (contaminants) within chamber 400 that are tobe detected are suitably removed or eliminated such that the absorptionspectrum of the sample obtained through use of device 10 is accurate asto the amount or presence of those gases (contaminants) within the gassample contained within system 400A. In accordance with a preferredaspect of the present invention, chamber 400 advantageously evidences asealed container which can be either purged of gas(es) (contaminant(s))to be detected, or evacuated to remove gas(es) (contaminant(s)) to bedetected, or in which the level of gas(es) (contaminant(s)) canotherwise be reduced below the level to be detected in the sample system400A.

Referring to FIGS. 2 and 3, in accordance with a preferred embodiment ofthe present invention, ILS chamber 400 (excluding the sample system400A) suitably comprises a container base 401 and attachable top 410.Respective windows 402,403 are suitably positioned in the walls of body401 in a suitable manner and position relative to the optical resonatorcavity defined therewithin. Container base 401 and top 410 suitablycomprise stainless steel or aluminum. Top 410 is advantageously securedto body 401 in accordance with any conventional technique suitable topermit evacuation, purging and/or further removal of contaminantstherewithin. For example, a gasket 410A or other suitable means togetherwith sealing devices (e.g. mechanical assists, adhesives and the likeall not shown ) may be employed for such purposes.

Windows 402,403 are suitably disposed in the walls of container 401,thereby providing optical access to ILS chamber 400. Preferably windows402,403 comprise optical windows with such as those available as modelsP910125 and P117125 from ESCO of Oak Ridge, N.J. Preferably, window 402is suitably provided with an antireflective (AR) coating on the order ofabout 1000 nanometers (nm) about 1100 nm, and optimally about 1064 nm.On the other hand, window 403 preferably comprises an optical windowwithout an AR coating. Window 402 is suitably designed to provide formaximum transmission at 1064 nm. Similarly window 403 is suitablydesigned to provide maximum transmission over the operational wavelengthregion of the ILS laser 500 (e.g. 1350 nm to about 1550 nm).

In such cases where the contaminant comprises water vapor, it isnecessary that water levels in chamber 400 be reduced below those whichare contained within the sample. In accordance with the presentinvention, detection levels of up to 10 parts per trillion (ppt) areobtainable. While any now known or hereafter devised method for removingcontaminants (for example water) from chamber 400 (excluding the samplesystem 400A) can be practiced within the context of the presentinvention, preferably, chamber 400 is appropriately sealed and inertgases, such as nitrogen are pumped therein. In some instances it may benecessary to further evacuate the chamber so as to create a vacuum whichremoves substantially all contaminants contained therein. In accordancewith yet a further aspect of the present invention, a getter (not shown)may be advantageously employed with chamber 400 to provide even furtherelimination of water within chamber 400. As will be appreciated by thoseskilled in the art, a getter (e.g. a molecular sponge) having thecapacity for continuously absorbing water may be utilized to reduce thelevel of water (contaminants) below the water concentration that is tobe detected in the gas sample cell 404-406 (e,g., 10 ppt). Aparticularly useful getter comprises model PS3N13N1 available from thePall Corporation of City of Industry, Calif.

As briefly mentioned above, ILS sensor 500 suitably optically detectsgaseous species (contaminants, e.g., water vapor) contained in a sampleplaced within chamber 400. In accordance with the present invention, ILSlaser 500 suitably comprises a crystal 507 mounted in a crystal holder508. Crystal 507 is suitably mounted in crystal holder 508 such thatcrystal 507 also is optimally placed with reference to the incomingbeam. As previously briefly mentioned, the incoming beam is suitablyshaped either by operation of pump 100 or through use of beam shapingassembly 200 such that incoming beam D suitably matches the mode volumeof the ILS gain medium (e.g. crystal 507). In general, ILS laser 500 issuitably configured such that the laser beam in the intracavity regionis substantially parallel (i.e., astigmatically compensated) in theregion where the beam is directed to gas sample, e.g. as containedwithin system 400A. While a variety of optical configurations may beemployed for this purpose, these mirror configurations have been foundto be particularly advantageous. Such a configuration permits theaccurate astigmatic compensation of the incoming beam thus permittingsimultaneous meeting of the optical conditions necessary to pump ILSlaser at the lasing threshold and generation of a laser beam which issubstantially parallel as it is directed to the gas sample, such ascontained within system 400A.

In accordance with this aspect of the present invention, respectivemirrors 501,505 and a folding mirror 503 are suitably employed for thispurpose. Mirror 501 preferably comprises an optical mirror having an ARcoating between about 1000 nm and 1100 nm, optimally 1064 nm. Mirror 501has a coating that effectively provides on the order of about 99.8% toabout 100% reflectivity in the desired spectral region (e.g. about 1350to about 1550 nm). Suitably, mirror 501 evidences a radius of curvature(ROC) of about 10 centimeters and evidences a diameter on the order ofabout 1.0 to about 1.30 cm, optimally about 1.27 cm. Preferably, mirror501 comprises a mirror available from Rocky Mountain Instrument Co. ofLongmont, Colo.

Preferably, mirror 503 comprises a folding mirror which is configuredsimilarly to mirror 501. In accordance with a preferred aspect of thepresent invention, mirror 503 has a coating suitable to achievereflectivity of about 99.8% to about 100% over the desired spectralregion (e.g. about 1350 nm to about 1550 nm). Mirror 503 suitablyevidences a diameter on the order of about 1.0 to about 1.3 cm,optimally about 1.27 cm.

Preferably, mirror 505 comprises a flat mirror (ROC≈∞). With referenceto FIG. 2, side 505A of mirror 505 facing mirror 503 is advantageouslyprovided with a reflective coating between 1350 nm and about 1550 nm.The other side 505B of mirror 505 is suitably uncoated. Preferablysurfaces 505A and 505B ate suitably wedged one against the other at anangle of on the order of about 0.5 to about 3.0 degrees, optimally about1.0 degree. The present inventors have found that wedging such surfacesin this manner tend to minimize undesirable reflections which may leadto interference effects.

Mirrors 501,503,and 505 are suitably mounted in respective mirror mounts502, 504, and 506. While any mount which is configured to maintain themirror in place may be utilized, suitable mounts include NEW FOCUS 9800series mounts which are available from Newport/New Focus of Sunnyvale,Calif.; high vacuum compatible mounts NEW FOCUS model 9581 areparticularly preferred. Mounts 502, 504, and 506 also enable mechanicaladjustment to optically align the ILS cavity within chamber 400. Forexample, the present inventors have found that the efficiency of the ILSlaser 500 is optimized for a laser configuration in which mirrors 501and 505 are aligned for a beam incident angle of about 0° and mirror 503is aligned for a beam incident angle of about 12.5°. Through theappropriate design, placement and configuration of mirrors 501,503 and505 beam D is substantially parallel (i.e. collimated) in the regionbetween mirrors 503 and 505. As a result, sample system 400A can beinserted within the intracavity region without significant deleteriouseffect in the performance of ILS laser 500. While certain tolerances areof course permitted in the design and arrangement of mirrors 501,503 and505, without astigmatic compensation, ILS laser 500 may becomemisaligned or operate in an uncontrolled fashion.

ILS laser crystal 507 preferably operates in a wavelength regionsuitable for detection of the contaminants contained within the gassample (e.g. water vapor) over which a signature absorption spectrum canbe obtained. As previously mentioned, laser crystal 507 generallyexhibits the properties of a multimode laser system. Light produced bylaser crystal 507 typically has a mode spacing of about 450 megahertz(MHz) to about 550 MHz, preferably about 500 MHz, thus ensuring accuratespectral replication of absorption bands. While any crystal may beutilized in the context of the present invention, a Cr⁺⁴ :YAG or Cr⁺⁴:lutetium (Lu)AG laser crystal may be optimally used in connection withthe present invention. Moreover, other hosts for the Cr⁺⁴ ions may beused and other doped ions into these host crystals may be substituted.The present inventors have found that the low gain efficiency of a Cr⁺⁴system in a garnet host (e.g. YAG or LuAG) can be operated successfullyas a laser using a crystal that is on the order of about 10 mm to about30 mm in length and 5 mm in diameter. Preferably the doped concentrationis in the range of about 0.10% to about 0.30% in such garnet host. Aparticularly preferred laser medium comprises a Cr⁺⁴ :YAG or Cr⁺⁴ :LuAGcrystal cut at Brewster's angle to minimize reflective losses. Moreparticularly a Cr⁺⁴ :YAG crystal cut at Brewster's angle (e.g. for acrystal refractive index where n is about 1.82 and ⊖_(B) is about 61.20)having a crystal length of about 23 to about 27 mm at about a 0.15%dopant level has been found to be particularly advantageous in thecontext of detector 10 in accordance with the present invention.

Laser crystals currently available, while improving in efficiency, haveconsiderable losses associated with them. The losses translate to heat.In accordance with the present invention crystal 507 suitably is mountedin a manner allowing for the effective removal of the heat thusgenerated in operation. It should be appreciated, however, that as theefficiency of laser crystals continue to improve as new crystals aredeveloped, the need or requirements on heat removing devices will bereduced and likely, at some point, the losses will be small enough thatthe need to remove the heat may be eliminated all together. However,using crystals presently available and in accordance with variousaspects of the present invention, ILS laser system 500 preferablyfurther comprises a heat sink system 500A.

With continued reference to FIG. 2 and additional reference to FIG. 5,heat sink system 500A desirably is operatively connected to mount 508and crystal 507 (not shown in FIG. 5). As shown best in FIG. 5, holder508 preferably comprises a two-part holder 508 suitably arranged tosnugly maintain crystal 507 in an operative arrangement. Heat sinksystem 500A preferably comprises a copper heat sink bridge 510, athermal electric cooler 509 and a thermal electric sensor 511. Bridge510 preferably comprises oxygen-free copper. Thermal electric cooler 509preferably comprises model CP1.0-71-05L available from Melcore ofTrenton, N.J. Thermal electric sensor 511 preferably comprises an Omegatype 100W 30 Platinum (RTO) resistant temperature detector. Mount 508together with bridge 510, cooler 509 and sensor 511 serve to properlyalign crystal 507 with respect to the other optical elements comprisingILS laser 500, as well as enable control of the thermal properties ofthe crystal.

In accordance with the preferred aspect of the present invention, heatsink system 500A is in direct physical contact with crystal 507. Heatproduced by normal operation of crystal 507 through optical excitationoccasioned by beam E is effectively conducted away from crystal 507thereby maintaining a relatively constant operating crystal temperature.Preferably, crystal holder 508 comprises copper/aluminum which isoperatively connected to cooler 509 and heat sink bridge 510. Suitably,heat sink 510 comprises a copper heat sink located in body 401 ofchamber 400 such that excess heat is conducted away from crystal 507.Sensor 511 measures the temperature of holder 508, cooler 509, bridge510 and crystal 507 such that optimum operating temperatures aremaintained. In accordance with this aspect of the present invention,thermal management of crystal 507 is obtained, thereby eliminating theneed for coolant liquids which may unnecessarily compromise andcomplicate the operation of detector 10.

ILS laser system 500 is suitably arranged such that the angle (φ) of thebeam exciting crystal 507 and the reflected beam from mirror 503 is onthe order of about 20° to about 30°, more preferably from about 23° toabout 27°, and optimally about 25°. This beam may suitably be directedto the sample system 400A such that detection of contaminants within thesystem may readily be obtained.

Output beam A from ILS laser 500 after passing through sample system400A is suitably directed to spectrometer 600. Such direction can beobtained, such as shown in FIG. 2, through use of a folding mirror 601suitably mounted in a mirror mount 602. Mirror 601 preferably comprisesa plane mirror containing a coating for high reflectivity in the desiredspectral region (e.g. 1350 nm to 1550 nm). In accordance with aparticularly preferred embodiment of the present invention comprisesmodel 5103 available from New Focus of Sunnyvale, Calif.

With continued reference to FIG. 2, spectrometer 600 suitably comprisesdispersive gratings designed to spectrally resolve a coherent beam, suchas the absorption spectrum of the contaminant in the sample to bedetected. Suitably, the spectral dispersion of the spectrometer issufficiently large to clearly resolve the absorption features of suchcontaminant, thus enabling the identification of the "signature" of eachcontaminant and the quantitative determination of the concentration ofthe contaminant. While any now known or hereafter devised spectrometermay be utilized in accordance with the present invention, preferablyspectrometer 600 comprises two diffraction grating assemblies 600A and600B operating in conjunction with an optical beam expanding assembly600C and a focusing lens assembly 600D. Optical beam expanding assembly600C preferably comprises lenses 603 and 605 suitably mounted withindetector 10 through use of mounts 604 and 606. Lens 603 preferablycomprises a negative lens with an AR coating at between about 1000 nmand 1500 nm, optimally 1400 nm; lens 605 preferably comprises acollimating lens with an AR coating at about 1000 nm and 1500 nm,optimally 1400 nm. In accordance with a particularly preferred aspect ofthe invention, lens 603 comprises a model KBC013,AR.18 available fromNewport of Chicago, Ill. and lens 605 comprises a model KBX115,AR.18available from Newport of Chicago, Ill.

Diffraction grating assemblies 600A and 600B suitably compriserespective diffraction gratings 607 and 609 mounted on respectivediffraction grating mounts 608 and 610. As will be appreciated, mounts608, 610 permit tuning and adjustment of diffraction gratings 607, 609within spectrometer 600. In this regard, and in accordance with apreferred aspect of the present invention, mounts 608 and 610 suitablyinclude respective adjustments that enable the efficient and accuratetuning and adjustment of diffractions gratings 607 and 609.

Focussing lens assembly 600D preferably comprises a lens 611 havingdimensions on the order of about 50.8 nm by 50.8 nm, containing an ARcoating at about 1000 nm and 1500 nm, optimally 1400 nm. Lens 611 issuitably positioned within a lens holder (not shown) which also enablesits accurate placement and adjustment within spectrometer 600.

In accordance with a preferred aspect of the present invention and withcontinued reference to FIG. 2, the spectrally-resolved ILS absorptionspectrum produced by spectrometer 600 is suitably displaced spatiallyacross a focal plane located at a fixed distance from lens 611. Inaccordance with a particularly preferred aspect of the presentinvention, a multichannel array detector 701 is suitably fixed on amount 702. An electronic board 703 containing the control and timingelectronics required to operate and read information from themultichannel detector 701 is operatively connected to detector 701.Detector 701 is suitably located in the optical beam exiting thespectrometer 600 such that it is sensitive over the region which the ILSlaser operates. As a result, the entire spectrally dispersed absorptionspectrum of the particular contaminant sought to be identified throughuse of detector 10 can be obtained. As will be described in greaterdetail below, the positions and relative intensities of the specificabsorption features of the contaminant can be utilized to uniquelyidentify the detected gas (contaminant) as well as quantitativelydetermine the amount of the gas (contaminant) so detected.

The light detected by detector 701 is preferably transduced intoelectronic signals at each detector element (pixel) with signalsthereafter transferred to an analog-to-digital (A/D) converter 801through board 703. Converter 801 is suitably connected through a BNCconnector and shielded cable 704 such that the accurate transfer ofinformation is ensured. As will also be described in greater detailbelow, once the data is so converted, it is manipulated by a computer802 which may be suitably programmed to convert the electronic signalsinto spectral information i.e. spectral signatures identifying aparticular gas (contaminant) and concentration of gases (contaminants).Detector 701 preferably comprises an InGaAs multichannel (256 pixel 100μm spacings) array detector. For example, model J18M-FP-100X 5OU:256Efrom E.G.&G. of Sunnyvale, Calif. or model SU256L-17T-1-100B fromSensors Unlimited of Princeton, N.J. may be advantageously employed inaccordance with the present invention.

With reference now to FIGS. 2, as previously mentioned, detector 10 iscaused to operate once pumping laser 100 causes ILS laser 500 to operateat or near its threshold level. In cases where the opticalcharacteristics of driver 100 suitably match those of ILS laser 500, noadditional modification of the output of driver 100 is necessary.However, in those cases where the remote volume of the pumping radiationof driver 100 does not suitably match the gain medium of ILS laser 500,beam modification system 200 can be utilized to facilitate suchmatching. In its simplest form, beam modification system 200 preferablycomprises an optical telescope useful to optimize the radiationdelivered to ILS laser 500 by focusing the required photon density intothe correct location and volume of the gain medium of ILS laser 500.Specifically, beam modification system 200 is used to alter the pumpingradiation of driver 100 to meet the requirements of laser 500.

In accordance with a particularly preferred aspect of the presentinvention, beam modification system 200 comprises a beam expandingtelescope. Specifically, in such cases where pumping laser 100 comprisesthe preferred Spectra Physics diode pump and ILS crystal 507 comprises aCr⁴⁺ :YAG or a Cr⁴⁺ :LuAG crystal, beam expansion of the pumping laseroutput is necessary.

As shown best in FIG. 4, system 200 suitably comprises a series oflenses and adjustable apertures. In particular, a frame 201 is providedwhich suitably includes respective upstanding walls into whichrespective variable aperture devices 202 and 205 are suitably provided.While any appropriate means of varying the aperture of the opening intowhich the output (e.g. beam E) of laser 100 enters or exits system 200may be employed, suitably, devices 202 and 205 comprise conventionalaperture varying devices such as model ID-0.5 available from Newport ofChicago, Ill. System 200 also preferably includes a focusing lens 203and a collimating lens 204. Preferably, lens 203 comprises a focusinglens with an AR coating at about 1000 nm to 1100 nm, optimally about1064 nm; lens 204 preferably comprises a collimating lens with an ARcoating at about 1000 nm to 1100 nm, optimally about 1064 nm. As shownin FIG. 4, lenses 203 and 205 are suitably attached to frame 201 topermit their respective alignment within beam E.

With continued reference to FIG. 2, in some applications, it may benecessary that the incoming beam be appropriately focused into the lasermedium (e,g., ion-doped crystal or glass) 507 within ILS laser 500. Inaccordance with the preferred aspect of the present invention, afocusing lens 206 may be advantageously mounted in a laser lens mount207 such that lens 206 is suitably located within the path of beam F. Inaccordance with a particularly preferred aspect of the presentinvention, focusing lens 206 suitably comprises an optical focusing lenswith an AR coating of between about 1000 nm and about 1100 nm andoptimally 1064 nm coating. In accordance with this aspect of the presentinvention, lens 206 is suitably selected such that it has a focal lengthin the range of about 10 centimeters (cm) and a diameter on the order ofabout 2.54 cm. Focusing lens 206 may be suitably configured to evidencea plano-convex or convex-convex configuration. A particularly preferredoptical focusing lens comprises model KPX094,AR.18 available fromNewport of Chicago, Ill.

As will be appreciated by those skilled in the art, the quality of thequantitative information obtainable through use of detector 10 depends,at least in part, on stable operation of ILS laser 500. In the contextof the present invention, the stability of laser 500 depends directly onhow reproducibly ILS laser 500 reaches threshold. Desirably, pumpinglaser 100 suitably pumps ILS laser 500 continuously near threshold whereits greatest sensitivity may be obtained. However, not all drivers arecapable of reliably operating in a continuous fashion. In addition,operating continuously tends to require substantial effort to maintainamplitude and wavelength stability which may have an adverse impact oncost and thereby produce an adverse impact on the commercial viabilityof detector 10. As an alternative to operating ILS laser 500continuously, and in accordance with a preferred embodiment of thepresent invention, ILS laser 500 is operated in a so-called "choppedmode". As used herein, the term "chopped" mode refers to the process ofreproducibly exposing ILS laser 500 to pumping radiation and, then,periodically sampling the output of ILS laser 500 which contains theabsorption information. Through operation in a "chopped mode", a stablelaser operation consistent with the quantitative spectral andconcentration measurements may be obtained in a commercially viablemanner. While any now known hereafter devised manner of producing achopped mode can be utilized in accordance with the present invention,advantageously such mode is obtained through use of modulation assembly300.

Referring again to FIG. 2, in accordance with a preferred embodiment ofthe present invention, modulation assembly 300 comprises modulator 301(provided with appropriate electronic circuitry) and modulator 304 (alsoprovided with appropriate electronic circuitry). Modulator 301advantageously modulates the intensity of output beam E from pump laser100 and lens 203, while modulation device 304 suitably modulates theoutput beam of ILS laser 500 that exits chamber 400, therebyperiodically sampling the output of ILS laser 500. Suitably, modulator301 alternatively blocks pumping beam E from reaching ILS laser 500 gainmedium (e.g., crystal 507), while modulator 304 alternatively blocks ILSlaser beam exiting chamber 400 from reaching both spectrometer 600 anddetector 700. Such intensity modulation (e.g. interruption) can beachieved utilizing, among other things, a mechanically operated chopper,an acousto-optic modulator, a shutter and the like. Alternatively, theoutput intensity of laser 100 may be modulated instead of secondarilychopping the output beam. Desirably, modulating device 300 does notsteer the pumping beam and is synchronized to modulate the intensity ofthe ILS laser output beam exiting chamber 400.

While specific form and placement of modulation assembly 300 isvariable, use of modulation assembly 300 enables generation of areproducible, effective optical path length within ILS laser 500. Statedanother way, by varying the generation time (t_(g)), i.e. the timeperiod over which intracavity mode competition within ILS laser 500 ispermitted to occur, the effective absorption path length within theintracavity resonator can be controlled and selected to achieve optimumquantitative application of detector 10.

In accordance with this aspect of the present invention, beam E isperiodically prevented from reaching ILS laser 500 by rotating modulator301 which periodically blocks and transmits the pumping laser beam E.Preferably, modulator 301 comprises a mechanical chopper such as BEMModel 350 available from Boston Electronics of Brookline, Mass.Mechanical chopper 301 is suitably placed so that beam E is modulatedbefore reaching ILS laser 500. While chopper 301 may be advantageouslyplaced before or after beam modification system 200, in accordance witha preferred embodiment of the invention, chopper 301 is suitably placedat the focal point within beam modification system 200. As bestillustrated in FIG. 2, chopper 301 is suitably mounted in frame 201 ofthe beam modification assembly 200.

It should be appreciated that chopper 301 could be replaced by anydevice, e.g., mechanical or electro-optical, which periodically blocksor modulates the pumping laser beam. As previously mentioned, inaccordance with the present invention, the intensity of the pumpingradiation emanating from laser 100 must only fall below that required tomake ILS laser 500 reach threshold and therefore, is not required toreach a zero value.

Suitably, chopper 301 rotates between an open and closed position.Chopper 301 is suitably driven by a chopper driver 303 (e.g. model 350Cavailable from Boston Electronics of Brookline, Mass.) connected tochopper through use of a suitable electrical connector(s) 302 (e.g.model A9049-ND available from Digikey of Theif River Falls, Minn.). Asdriver 303 causes chopper 301 to rotate to the open position, beam Ereaches ILS laser 500, thereby bringing ILS laser 500 above thresholdfor laser activity. ILS laser 500 continues to operate until driver 303causes chopper 301 to rotate to the closed position, whereupon chopper301 effectively blocks pumping beam E from reaching ILS laser 500. ILSlaser 500 output exiting chamber 400 is suitably directed to modulator304. In accordance with various aspects of the present invention,modulator 304 comprises an acousto-optic modulator, such as modelATM80A1 available from IntraAction of Bellwood, Ill. It should beappreciated, however that other available devices, for example, anothermechanically operated chopper or even a shutter may be suitably employedfor this purpose. As discussed above, to extract quantitativeinformation from the ILS laser 500 exiting beam, modulator 301periodically samples the output of ILS laser 500 which contains theabsorption data of contaminants (e.g. gaseous species) contained in theparticular sample. Suitably modulator 304 samples the output anddeflects it to spectrometer 600 and detector 700 for analysis.

Modulating device 301 is advantageously synchronized with modulationdevice 304 such that quantitative information from ILS laser 500 can beextracted in a time-resolved manner. Pump radiation E is effectivelydelivered to ILS laser 500 intermittently by passing pump beam E throughchopper 301. Delivering radiation intermittently alternatively bringsILS laser 500 near threshold and below threshold. After the generationtime, t_(g), elapses as ILS laser 500 nears its threshold, ILS laser 500output is deflected by modulator 304 to the entrance of spectrometer 600and detector 500 for detection. However, ILS laser 500 output beam G isdeflected to spectrometer 600 and detector 500 for only a short timeinterval determined by the synchronization of modulation devices 301 and304. The synchronization of modulators 301 and 304 ensures thatradiation from ILS laser 500 is sampled over a well-defined timeinterval (t_(g)).

Synchronization of modulators 301 and 304 may be achieved by severalconventional methods such as, for example, through electronic control bya digital circuit (not shown) operated by computer 802 operativelyconnected to detector 10. Typically, synchronization of modulators 301and 304 will be suitable to generate generation times (t_(g)) on theorder of less than about 300-500 μsec, more preferably on the order ofless than about 10-100 μsec, and optimally on the order of less thanabout 1 μsec. Such synchronization results in chopper 301 being open ata time when modulator 304 is closed and the time interval between whenmodulator 301 open and modulator 304 opens determined by t_(g).

In accordance with a preferred embodiment of the present invention, amethod for detecting the presence and concentration of contaminants in agas utilizing detector system 10 suitably comprises reducing gases(contaminants) in sample system 400A to an acceptable level, placing asample of gas to be detected in sample system 400A, pumping ILS laser500 at or near threshold, periodically sampling the optical output fromILS laser 500, preferably via modulation assembly 300, measuring theabsorption spectrum of the gases (contaminants) within the sample withspectrometer 600 and detection system 700, and analyzing the absorptionspectrum to identify the gaseous species (contaminants) and determineits concentration within the sample utilizing computer/software system800.

More particularly, reducing gases (contaminants) in chamber 400(excluding sample system 400A) to an acceptable level may suitablycomprise purging or evacuating sealable container 401 with top 410 suchthat the level of gases (contaminants) is below that to be detected inthe gas sample within system 400A. As discussed previously, othermechanisms for reducing the level of gases (contaminants) may beutilized provided they can reduce the level to an acceptable level.Preferably, container base 401 is sealed to top 410 and contaminantscontained therein are effectively removed (or reduced to an acceptablelevel). Desirably, base 401 and top 410 are effectively sealed prior todelivery to a user in a relatively tamper-proof manner.

A sample is suitably communicated to system 400A by connecting a gasline to connectors 408, 409 and feeding the gas into sample system 400A(for example, when the sample comprises a corrosive gas), or into thechamber itself (for example, when the sample comprises a non-corrosivegas).

Pumping ILS laser 500 at or near threshold more particularly comprisesselecting the correct pump laser 100 power, focusing conditions at lasercrystal 507 utilizing beam modification optics 200 and lens 206, andmodulation conditions utilizing modulator system 300. The method fordetecting gaseous species in accordance with the present inventionfurther comprises driving ILS laser 500 at or near threshold. Inaccordance with the present invention, driver 100 suitably pumps ILSlaser 500. Where necessary, pumping beam E is suitably shaped by beamshaping assembly 200 to meet the optical requirements of ILS laser 500.Further, where detector 10 is operated in a chopped mode, as describedabove, modulation assembly, and in particular, modulator 301periodically interrupts pump beam E thereby preventing beam E fromreaching ILS laser 500. Beam F output from modulator 301 and beamshaping assembly 200 is suitably directed to ILS laser 500.

In accordance with this method, as beam F enters chamber 400 throughwindow 402 disposed in the wail of sealed container body 401, beam F issuitably directed to ILS laser 500. Additional focusing and direction ofbeam F may suitably be achieved as beam F passes from window 402 tofocusing lens 206, where focusing lens 206 suitably focuses beam F anddirects it through mirror 501. Beam F suitably pumps crystal 507 at ornear threshold, and the output beam is suitably directed to the gassample within system 400A, such as by mirrors 503 and 505. The exitingbeam, containing the absorption data from the gas (contaminant) sample,then exits gas chamber 400 through window 403 suitably disposed in awall of sealed container body 401.

Preferably, where ILS laser 500 is operated in a chopped mode, modulator304, suitably synchronized to modulator 301, periodically samples ILSlaser 500 output beam and passes the sample thus obtained tospectrometer 600 and detector 700. Suitably, mirror 601 directs sampledILS laser 500 output beam G to spectrometer 600 and detector 700.

The method for detecting gaseous species in accordance with the presentinvention further comprises analyzing the sampled ILS laser 500 outputbeam G. Preferably, spectrometer 600 spectrally resolves and detector700 suitably analyzes sampled ILS laser 500 beam G. Spectrometer 600suitably spectrally disperses ILS laser 500 beam G through beamexpanding assembly 600C, diffraction assemblies 600A, 600B and focusingassembly 600D. Spectrally-resolved ILS absorption data exitingspectrometer 600 is suitably displaced spatially to be detected bymultichannel detector 701.

With reference now to FIGS. 6 and 7, detector 10 can be utilized toobtain absorption spectrum for contaminants, such as water vapor, incorrosive (e.g. HCI) or non-corrosive (e.g. N₂) over a variety ofwavelength regions. As shown in FIG. 6, showing a plot of normalizedlaser intensity/absorption vs. wavelength over the region of about 1433nm to about 1440 nm, and in FIG. 7, showing a similar plot over thewavelength region of about 1420 nm to about 1434 nm, signatures of awater vapor in each environment can be obtained through operation ofdetector 10. The spectrum displayed in FIGS. 6 and 7 were obtained byscanning of spectrometer assembly 600 under control of computer 802 andmeasurement of the output of diode 701. As will be appreciated, eachdata point illustrated results from a variety of measurements.

Given the relationship between intensity and concentration, once acharacteristic signature of the contaminant gas, e.g. water vapor, isobtained, the concentration of the contaminant contained within thesample can be readily obtained. While in accordance with the presentinvention computer 802 is suitably programmed to interpret the data andprovide an output indicative of the presence and/or concentration of thecontaminant contained within the sample, representative plots asobtained through operation of detector 10 showing the high sensitivitydetector possible through use of the present invention illustrativelyshown in FIG. 8 and 9. In FIG. 8, detector 10 is used as a permeationdevice; in FIG. 9, detector 10 is used as an in-line purifier.

Those skilled in the art will appreciate that the detection levelsavailable through practice of the present invention generally exceedthose which are obtainable through use of conventional devices.Moreover, detector 10 can be used in-line and obtain ready, nearreal-time measurement of the presence and amount of the contaminantcontained in a specific sample, thus addressing the many disadvantagesassociated with the use of such conventional devices.

It should be understood that the foregoing description relates topreferred exemplary embodiments of the invention, and that the inventionis not limited to the specific forms shown herein. Various modificationsmay be made in the design and arrangement of the elements set forthherein without departing from the scope of the invention as expressed inthe appended claims. Moreover, the application of detector 10 as well asthe location of detector 10 in, for example a semiconductor fabricationassembly, can vary as may be desired. For example, the specificplacement of the various elements within the ILS chamber and detector 10itself may be modified so long as their configuration and placementsuitably enables optical excitation of ILS laser 500 in a readilyreproducible manner. These and other modifications in the design,arrangement and application of the present invention as now known orhereafter devised by those skilled in the art are contemplated by theamended claims.

I claim:
 1. A detection system for detecting the presence of acontaminant in a sample, comprising:a chamber suitably configured to beevacuated of contaminants to be detected in the sample, said chamberincluding therewithin an intracavity laser comprising a laser crystalcontained within an intracavity resonator, and a sample system forcontaining the sample suitably placed within an astigmaticallycompensated region of said resonator; a spectrometer assembly; and adetector assembly suitably connected to a computer system programmableto interpret the spectral resolution of said spectrometer assembly andprovide an output indicative of the presence and/or concentration ofcontaminant in the sample.
 2. The detection system of claim 1 includinga driver having an output that pumps said laser crystal.
 3. Thedetection system of claim 2 including beam shaping optics locatedoutside said laser cavity that shapes said output of said driver.
 4. Amethod for detecting contaminants contained within a gaseous samplecomprising the steps of:directing the output beam of a driver to a lasercrystal contained within an intracavity resonator contained within anevacuable/purgable chamber to excite said laser crystal near a thresholdlevel; providing a sealed sample system for containing the sample to bedetected in an astigmatically compensated region of the intracavityresonator in which said laser crystal is maintained; directing theoutput energy from said laser crystal to said sample; resolving thespectral absorption of said laser output after interaction with saidsample to determine the presence of and/or amount of any contaminantswhich may be contained within the sample.
 5. The method of claim 4wherein said output beam of said driver is chopped.
 6. The method ofclaim 4 wherein said output energy from said laser crystal afterinteracting with said sample is directed to a detector assembly fordetermining the presence of and/or amount of said contaminants.
 7. Themethod of claim 6 wherein said output energy from said laser crystalafter interacting with said sample is alternately prevented fromreaching said detector assembly.